Acoustic wave devices using piezoelectric thin film resonators have been used as filters and multiplexers of wireless devices such as, for example, mobile phones. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film as disclosed in, for example, Japanese Patent Application Publication No. 2014-161001. The region where the lower electrode and the upper electrode face each other across the piezoelectric film is a resonance region.
A capacitor may be connected in series with or in parallel to the piezoelectric thin film resonator. Since the piezoelectric thin film resonator functions as a capacitor at frequencies away from the resonant frequency, it may be considered to use the piezoelectric thin film resonator as a capacitor. However, the piezoelectric film of the piezoelectric thin film resonator is thick, resulting in increase in area of the capacitor.